This invention relates to a method of suppressing feedback oscillations in free electron lasers.
This invention was made with Government support under Grant No. PHY83-06077 awarded by the National Science Foundation, and Contract No. N00014-80-C-0062 awarded to the Office of Naval Research. The Government has certain rights in this invention.
Free electron lasers are currently under investigation as a promising source of high power microwave and millimeter wave radiation. The free electron laser produces coherent radiation by subjecting a cold, intense electron beam to a periodic "wiggler" magnetic field which causes the electron beam to oscillate transversely, and this periodic oscillation couples via the ponderomotive force to a longitudinal plasma oscillation. The net induced oscillating current drives the coherent output radiation with the energy for the radiation coming at the expense of the beam kinetic energy. The outstanding properties of the free electron laser include not only high efficiencies and output powers but also its inherent frequency tunability. Unlike klystrons and traveling wave tubes, free electron lasers are fast wave devices and are not limited, by size constraints, to low power levels at high frequencies and, unlike gyrotrons, they are readily operable as an amplifier.
With traveling wave tubes, one of the major problems has always been oscillations caused by waves on the slow-wave interaction circuit flowing in a direction opposite to that of the signal being amplified. As with any such high gain amplifiers, free electron lasers are also subject to so-called feedback oscillations, that is, a small fraction of the output power reflected backward to the input and reamplified to produce regenerative gain and eventual oscillations. Observation of such oscillations in a free electron laser was reported, for example, in "Microwave Studies of a Tunable Free Electron Laser in Combined Axial and Guide Magnetic Fields" by J. Fajans, G. Bekefi, Y. Z. Yin and B. Lax published in Phys. Fluids 28, 1995-2006 (1985) when the wiggler magnetic field strength exceeded a certain threshold value and these oscillations precluded gain measurements above 20 dB. Use of such feedback oscillations to measure the free electron gain has been reported in "Gain Measurements from Start-up and Spectrum of a Raman Free Electron Laser Amplifier" by J. Masud, T. C. Marshall, S. P. Schlesinger and F. G. Yee in Phys. Rev. Lett. 56, 1567- 1570 (1986) and in "High Gain Long Pulse Free Electron Laser Oscillator" by J. Matthew and J. A. Pasour in Phys. Rev. Lett. 56, 1805-1808 (1986). The backward waves may be initially caused by reflection from a mismatched output circuit. At microwave and millimeter wavelength regions, it becomes difficult to design input and output couplers with voltage standing wave ratios (VSWR) less than 1.05. Consequently, gains above 30 dB are difficult to achieve and, at substantially lower gains, phase control is imprecise.